The present invention relates to an improved process for reducing or preventing the deposition of materials in a heat exchanger. More particularly, the process of the present invention relates to an improved process for reducing or preventing the deposition of naphthalene and tar in the ammonia liquor coolers in a process for coking coal.
Coke oven gas produced in the coking process for coal is cooled to produce condensed products such as an aqueous solution containing ammonia, tar, and light oil. In addition, the coke oven gas contains fixed gases such as hydrogen, methane, ethane, carbon monoxide, carbon dioxide and some unsaturated hydrocarbons.
As the coke oven gas leaves the coke oven, it is initially cooled with flushing liquor. The flushing liquor is primarily the water condensed from previously cooled coke oven gas and it contains ammonium salts. This initial cooling causes the condensation of a relatively heavy tar from the coke oven gas. The heavy tar and flushing liquor enter a decanter tank for separation. The non-condensed gases and vapors remaining in the coke oven gas after initial contact with the flushing liquor is cooled further to around 95.degree. F. (35.degree. C.) to remove additional tar and a major portion of water vapor in either direct or indirect primary coolers.
In the direct type cooler the gas is contacted with condensed water vapor containing free ammonia, hereinafter referred to as ammonia liquor. As a result of this second cooling, heat and water are removed from the gas, thereby reducing its volume while light tar with some naphthalene is condensed along with a considerable quantity of weak liquor containing ammonia. The light tar and ammonia liquor condensates are processed either separately or mixed with the heavy tar and flushing liquor at the flushing liquor decanter tank. The cooled coke oven gas can be compressed and passed through a naphthalene scrubber for the removal of naphthalene in order to prevent naphthalene from separating out of the gas downstream of the naphthalene scrubber. When the gas is cooled to a fairly low temperature in the primary cooler, naphthalene separation from the gas becomes a problem. Naphthalene is a solid at atmospheric temperatures and is negligibly soluble in water. Pure naphthalene melts at 176.degree. F. (80.degree. C.) and boils at 424.degree. F. (218.degree. C.). A considerable quantity of naphthalene condenses after the coke oven gas leaves the coke oven battery and dissolves in the heavy tar in the initial cooling step. However, even though additional naphthalene dissolves in the light tar phase present in the primary cooler, it can cause trouble in the primary cooler and auxiliaries, such as the ammonia liquor coolers, further downstream in the process.
In the indirect type primary cooler the gas flows through the shell part of a shell-and-tube heat exchanger in the primary cooler. The gas is cooled as it indirectly contacts water flowing countercurrently in the tube part of the heat exchanger. The presence of naphthalene in the gas is a definite problem. Most of the tar from the gas is condensed in the first section of the hotter end of the heat exchanger. Consequently, there is no tar in the last section on the cold end of the heat exchanger for dissolving naphthalene. Because of this, it is general practice to have several indirect primary coolers in parallel and always have one extra cooler.
After the gas mixture leaves the primary cooler, the gas mixture may flow into and through an ammonia scrubber. Finally, the cooled and semi-cleaned gas may next pass through a light oil scrubber or washer where wash oil contacts the coke oven gas and removes light oil from the gas. Further downstream from the light oil scrubber, there can be located a gas desulfurization absorber to remove hydrogen sulfide from the coke oven gas. If most of the ammonia has not been removed from the coke oven gas upstream of the gas desulfurization absorber, such ammonia will collect with the acid gases which contain mostly hydrogen sulfide, along with some hydrogen cyanide and carbon dioxide. This collection could cause polymerization problems between hydrogen cyanide and ammonia if a substantial quantity of ammonia is present in the gas.
To obtain both efficient removal of naphthalene from the coke oven gas at the naphthalene scrubber and efficient removal of ammonia at the ammonia scrubber as well as efficient recovery of light oil, requires relatively low gas temperatures, hence the need for effective gas cooling at the primary coolers. This effective gas cooling is effected by circulating to an indirect cooler the heated ammonia liquor which has been separated from the condensed tar after being used to cool the coke oven gas. Then the cooled ammonia liquor is conveyed back to the primary cooler to cool more coke oven gas and condense more water vapor and light tar.
The amount of naphthalene produced in the carbonization step is usually sufficient to practically saturate the light tar collected in the primary cooler. Since the ammonia liquor removed from the primary cooler and conveyed through the indirect coolers still contains traces of light tar, the naphthalene in the light tar is caused to crystallize out of solution on the liquor side cooling surfaces of the indirect cooler, when the liquor is cooled to desirable temperature for recycle to the primary cooler. This deposition on the cooling surfaces impairs the cooling efficiency of the indirect coolers. It also causes stoppages or impairs the flow of the liquor through the primary coolers. Therefore, this deposition has an adverse effect on the efficiency of cooling the gas, efficiency of naphthalene removal from the gas, efficiency of ammonia removal at the ammonia scrubbers, as well as the efficiency of light oil recovery.
In the prior art, there are many processes for more efficient removal of naphthalene from coke oven gas. Examples of these include the following U.S. Pat. Nos.: 2,710,663 (Wilson); 2,810,450 (Hartman); 3,000,693 (Schulte); and 4,001,347 (Grosick). These processes deal with naphthalene removal further downstream from the primary cooler for the gas and indirect cooler for the ammonia liquor, e.g., in the naphthalene scrubber and final cooler. These processes do not show a solution to the problem of naphthalene deposition in the indirect coolers which cool the ammonia liquor after it leaves the primary cooler, and before it is returned to the primary cooler to cool more coke oven gas.
A past remedy to prevent deposition of naphthalene on the cooling surfaces of the indirect coolers has been to add a mixture of light and heavy tar, sometimes referred to as production tar, which is collected in the flushing liquor decanter, to the ammonia liquor after the primary cooler and before the indirect ammonia liquor coolers to aid in dissolving the naphthalene that deposits on the cooling surfaces of the indirect cooler.
Unfortunately, modern coal mining methods are inclined to produce a coal that has a high percentage of extremely fine particles which escape from the battery of ovens with the coke oven gas and deposit in the tar in the flushing liquor decanter, resulting in a production tar that is high in quinoline insolubles. These insolubles exacerbate the separation of the tar from the liquor in the flushing liquor decanter and add to the deposition on liquor side surfaces of the indirect ammonia liquor coolers of the primary cooler. The amount of this tar that reaches the indirect cooler causes a sticky buildup of material that occludes to a degree the openings of the cooler. Because of the deposition of the quinoline insolubles or naphthalene or other compounds in the indirect cooler, it becomes necessary sometimes to restrict the flow of cooling waters to the indirect coolers or to temper the cooling water and thereby raise the temperature of the ammonia liquor leaving the indirect coolers and entering the primary coolers and, therefore, not cool the coke oven gas as much as desired. This production tar was preferred in the past because it had a low naphthalene content and it tended to prevent naphthalene deposition on the cooling surfaces in the cooler. But the practice of adding the low naphthalene, high quinoline insoluble tar to the ammonia liquor is so detrimental to the operation of the indirect coolers that the tar must be added to the liquor going to the primary cooler at some point after the indirect cooler. Such an addition is not the most facile or efficient manner for preventing the deposition of naphthalene and other materials in the indirect coolers for the ammonia liquor.
It is an object of the present invention to provide a process for minimizing the deposition of materials in an indirect cooler wherein ammonia liquor used to cool the coke oven gas is cooled before it is recycled to cool more coke oven gas.
An additional object of the invention is to maximize the efficiency of naphthalene removal, ammonia removal and light oil recovery in a coke oven by-product process by providing a process for effective cooling of the coke oven gas at the primary cooler.
A further object of the present invention is to provide a process to allow the most efficient use of an indirect cooler used to cool ammonia liquor that is used to cool coke oven gas.